Assay systems are now available, capable of detecting and quantitating trace amounts of bovine proteins (BGG, BSA, alpha- lactalbumin, beta-globulin, casein) in human serum and breast milk. The assays can be adapted to detect those proteins both as free molecules, or combined with antibody in circulating immune complexes. In addition, with the use of fluoresceinated reagents these proteins and/or their specific antibodies can be detected in tissue. The purpose of the proposed study is to utilize these techniques in an attempt to determine if bovine proteins (1) can cross the blood-gut barrier and enter the circulation as intact molecules and/or as digestion products: and (2) participate in the formation of potentially pathogenic immune complexes detectable in the circulation and/or the glomeruli of patients with immune complex nephritis. In additon, an attempt will be made to induce immune complex disease in mice by the daily feeding of various amounts of bovine protein. Other questions to be examined using the same techniques are: 1) is bovine protein ingested by a lactating mother secreted into the breast milk, and if so which proteins, and are they intact or digestion products: and 2) is the difference between milk sensitive individuals with positive skin tests and those without clinical sensitivity but with positive skin tests, related to differences in absorption of milk protein from the gastro-intestinal tract.